Tanks for the storage of liquids have been constructed in a variety of ways from a variety of materials. In one common application, the underground storage of hydrocarbons, such as gasoline and other petroleum products, the tanks have conventionally been fabricated out of steel or fiberglass, most commonly with a single rigid wall. In many applications this construction has proved reasonably satisfactory, with such tanks functioning properly for many years before requiring repair or replacement. However, the increasing age of many of the tanks currently in place is beginning to present serious environmental dangers. Many of the older steel tanks buried underground have rusted and are beginning to leak, thus releasing the petroleum materials into the ground where they may seep into and pollute underground water supplies. While rustproof, some fiberglass tanks have also exhibited leakage, causing the same problems.
One of the primary problems with leaking storage tanks has been the difficulty or inability to ascertain when or if such leaks are occurring from a given tank. Because the excavation and removal of such a storage tank, which may contain thousands of gallons of fuel, is an expensive and difficult undertaking, such an operation is difficult to justify unless there is some evidence of actual leakage.
Because of the increasing potential danger of leaking storage tanks, particularly in communities that utilize ground water for public consumption, many municipalities have implemented or plan to implement ordinances requiring the use of double wall storage tanks underground and requiring replacement of existing single wall tanks. While the installation of a conventional double wall tank in a new facility entails no great difficulty and a generally manageable increase in cost over a single wall tank, the burden of complying with such ordinances by replacing existing sound, single wall tanks with double wall tanks can be heavy. This burden has promoted the search for methods of fabricating relatively inexpensive double wall tanks. This burden has also given impetus to the search for a method of remanufacturing existing single wall tanks into a double wall assembly with means for detecting the presence of any leaks into the space between the two walls.
A number of techniques have been disclosed for converting single wall tank into double wall tank and providing such tanks with the means for detecting the presence of leaks. Such methods are shown for example in my U.S. Pat. Nos. 4,640,439, 4,644,627, 4,655,367, 4,744,137, 4,780,946, 4,780,947 and 4,817,817. Although these techniques provide for quite acceptable double wall tanks, they do require the employment of a perforated spacing material such as mesh, or alternatively, a molded spacing material that must be constructed and applied to the inner tank or alternatively the application of a release agent directly to the tank. The labor and time involved in constructing and installing these spacing materials or applying a separate coating of release agent can be significant and the need exists to reduce even further the cost of producing such double wall tanks.